This will unlock after completing the mission "The Last Battleship" without having your escort Battleships sustain enough damage to sink. Initially make sure you quickly destroy the kamikaze boats as they do massive damage to the battleships. At the end of the mission you'll encounter a large group of battleships, cruisers, and the Yamato. To end the mission you can quickly destroy the Yamato by yourself, or clear out the escort ships so your Battleships can help you take out the Yamato. Obviously this is much easier to do on Easy or Normal difficulty, especially since the addition of an Ace near the end of a level can make destroying the Yamato solo rather annoying.